The invention relates a process for applying a thin protective layer to the surface of optical plastic elements. Transparent plastic elements, which are in partifcular discussed here, can be shaped with a substantially less operating expense and with a substantially higher optical precision than corresponding glass elements. For example, it is possible to make plastic lenses for simple objectives or for spectacles in an economical manner by means of an injection molding process, whereby the aftertreatment of grinding and straight-line lapping, which is required with glass lenses, is eliminated. A further advantage consists in that more complex shapes with integrated additional functions can be made. For example, the cover for the main head lights and the blinking light of a motor vehicle can be made from one piece. It is disadvantageous in all plastic materials that the soft ductile material is subjected to a very great cutting wear, which is to be understood to mean in particular the erosion due to wear and tear with a predominant tangential component, for example, wiping off. Therefore, the surface must be protected by a well adhering, sufficiently hard cover layer which withstands in addition to the climate tests (ruling cutting and the tearing off of a standard splicing tape before and after the wet room test in accordance with SK (condensed water constant climate) DIN 50017, temperature change, etc.).
Mainly lacquers with different lacquer sytems are performed for the surface protection, whereby mainly acryl lacquer and polyorganosiloxane-lacquer are used. Thereby, the polyorganosiloxane-lacquer provide the better abrasion protection in relative comparison. The test for abrasion resistancy may be performed, for example, in accordance with an ECE-suggestion (Economic Commission for Europe) in the following manner:
A die of 14.times.14 mm is enveloped with a cloth and abrases with a contact pressure of 2 N/cm.sup.2 with a maximum speed of 3 cm/sec over the coating. A quartz powder with a Mohs-hardness of 7, grain size of 0 to 200 .mu.m is provied between the test face and the die in a substantial normal distribution and a corner ratio of 1.8 to 2.0. After 50 double strokes the stray light is measured in transmission in a space angle of 1.degree. to 12.degree.. A stray light value of 4% is provided as a maximum upper limit value in the ECE-standard suggestion. However, after this test the acryl lacquers have stray light values of more than 5% and the best polyorganosiloxane-lacquer only somewhat smaller than 4%.
Hard layers can also be made with PVD-and CVD-processes (Physical Vapour Deposition or Chemical Vapour Deposition). The CVD-processes require relative high process temperatures. Even with plasma supported CVD-processes the necessary process temperature is still too high, even for polycarbonate parts. Moreover, very brittle inorganic cover layers are generated in such CVD-reactions, like SiO.sub.2 or SiC, which do not adhere very well on the organic support and therefore chip off very often during the abrasion test. The same problem is encountered with quartz layers which are vaporized in a high vacuum (PVD-process). The hardness of such layers is very good. After the aforementioned stated ECE-abrasion test one can realize stray light values of smaller than 1%. However, in these layers the adherence on polycarbonate is very poor after a temperature change or temperature storage.
Furthermore, it is known that protective layers can be made by means of the glow polymer process Different silanes, siloxanes and silazanes are used as initial monomers. The polymer films which are separated in a glow discharge from these silicon organic compositions are relatively soft without any additional measures and do not offer any noticeable abrasion protection. However, the layer hardness increases when the number of the silicon-hydrogen-silicon compositions which are present in the polymer film is increased. This can be achieved in accordance with "Polymer Sci" (USSR) 9, 2281 (1967), for example, by adding argon-inert gas into a glow discharge which burns in a siloxane-monomer vapor, whereby hexamethyl disiloxane (HMDS) is used as a monomer.
In the "NASA Tech Briefs", Winter 1978, p.543 a similar process for making scratch proof layers on lenses made from PMMA is stated. Thereby, in accordance with the statements by the author an adhering support is made on the plastic surface in a first step by a treatment in a steam vapor-plasma by forming of hydroxyl-groups. In a second step the actual polymer film is applied made of a silicon organic monomer (in the example even parts of vinyl dimethyl ethoxysilane and hexamethyl disilicane). The most important third step in this context is the aftertreatment in an argon plasma. Thereby, in accordance with the statement by the author the netting of the polymer film is improved and its scratch resistance is increased.
The U.S. Pat. No. 4,085,248 describes a process for making lyophilic protective layers on the surface of optical reflectors. The lyophilisation of the polymer film which is formed from the silicon organic monomer hexamethyl disiloxane (HMDS) is achieved by an aftertreatment in an oxygen plasma. Thereby, more silicon-oxygen-silicon compositions are generated in the proximity of the surface areas of the polymer film. The increased limit tension which is obtained should also effect an improvement of the scratch resistancy. This was already noted in a treatise in "Applied Optics" 16, 717 (1977), whereby a certain improvement of the abrasion resistancy was achieved by means of an aftertreatment of a polymer film, which was made from a silicon organic monomer (vinyl trimethoxysilane), in an oxygen plasma. However, the protective effect for a polycarbonate face which is achieved therewith is not sufficient to pass the aforementioned ECE-test. Our own tests have shown that the resistancy against scratching of layers which were after-glowed in oxygen HMDS-layers were clearly poorer during the ECE-test than the one of the nontreated layers of a corresponding layer thickness.
In "Solar Energy Materials", 3, 301 (1980) it is suggested to introduce a certain component of oxygen into the monomer vapor during the separation of the polymer film for improving the scratch resistancy. Thereby, the oxygen partial pressure is smaller than the partial pressure of the monomeric evaporation. The number of the silicon-oxygen-silicon-compositions are increased in accordance with the publicised infrared spectrum, as is the case in the already discussed processes, and thereby also the layer hardness. However, despite the addition of oxygen the concentration of the methyl groups, which give an organic soft character to the polymer film, remain practically unchanged. Also, the sand falling abrasion method which is used in accordance with ASTM D 968-51 is a less stringent test method than the abrasion test by the ECE as aforementioned. Furthermore, in the cited treatise glass supports were used as a carrier substance for the layer to be applied, so that the problem of the layer adhering on a plastic substance has not to be considered.